1. Field of the Invention
The present invention relates to a method of continuously drawing a large-size elongate cable for installation.
2. Description of the Relevant Art
One conventional process of continuously laying an electric cable for installation is shown in FIG. 1 of the accompanying drawings.
As shown in FIG. 1, an electric cable 2 unreeled from a cable drum 1 of metal is drawn by a plurality of cable drawing machines 3 spaced at a certain interval over a plurality of cable rollers 6 disposed between the cable drawing machines 3. The cable drawing machines 3, which have caterpillars for advancing the electric cable 2, are actuated by respective induction motors 4 to feed the electric cable 2 over the cable rollers 6. The induction motors 4 are electrically connected to a power supply cable 7 through respective switches 5.
While the electric cable 2 is being withdrawn from the cable drum 1 by the cable drawing machines 3, the speed at which the electric cable 2 travels tends to vary depending on the load on the electric cable 2. Therefore, it has been customary for the cable drawing machines 3 to be attended by respective skilled operators 8 who turn on and off the switches 5 to adjust the cable speed at the respective cable drawing machines 3 while at the same time communicating with an operator who attends the cable drum 1.
Since the cable drawing machines 3 are actuated by the induction motors 4, they start to operate at full speed once the switches 5 are turned on. Consequently, it has been impossible to gradually increase or reduce the speed of operation of the cable drawing machines 3 to feed the electric cable 2. Fine adjustment of the speed of travel of the electric cable 2 has been effected manually by the skilled operators 8 who selectively turn on and off the induction motors 4 for movement of the electric cable 2 in small increments.
In the case where the electric cable 2 to be installed is of a relatively large size, the cable drawing machines 3 are spaced at an interval of at most 50 m to 60 m depending on the weight of the electric cable 2, the capacity of the cable drawing machines 3, and the capacity of the cable rollers 6. Because of a large voltage drop developed across the power supply cable 7, the electric cable 1 can be drawn over a distance up to about 1000 m in one cable drawing process.
In view of the above conventional drawbacks, there has been a demand for automatic centralized control of the drawing of an electric cable. It has also been desired to install an electric cable over a distance ranging from 5000 m to 10000 m continuously in one cable drawing process. The cable drawing machines should preferably be spaced at a distance of about 100 m for drawing the electric cable with higher efficiency, and the cable rollers should preferably have a minimum coefficient of friction.
When an electric cable is to be laid across a river or through a tunnel, it may sometimes be required to have a length in the range of from 3000 m to 5000 m. Since such a lengthy electric cable cannot be wound on a cable drum for shipment or transportation, it is wound horizontally and stored in a large-size container. To draw the electric cable from the container, the container is placed on a horizontal turntable, and the turntable is rotated in coordination with cable drawing machines spaced along a cable laying route.
The container is considerably large in size to house the lengthy electric cable, and the turntable has a considerably large weight. The total weight of the container, the electric cable, and the turntable may sometimes reach 100 tons or more. Therefore, the turntable needs to start and stop slowly when it is necessary to start and stop drawing the electric cable from the container. When a stop signal is applied in case of emergency, since the inertial forces of the turntable and the container are very large, the turntable has to be stopped after it has been braked for a certain period of time, i.e., until a certain cable length is reeled out, in order to avoid damage to various parts of the container, the turntable, and the cable drawing machines. This means that the electric cable must be loosened to form a slack therein which is commensurate with the difference between the inertia of the system of the container, electric cable, and turntable, and the inertia of the system of cable drawing machines. Consequently, it is necessary to continuously draw the electric cable while absorbing the slack without imparting undue twists and stresses to the electric cable. Heretofore, however, such a requirement has not been met by the conventional cable drawing method which has only laid the electric cable for a distance up to 1000 m.